The present invention relates to a polymer electrolyte, which can be used in electrochemical equipment such as cells, capacitors, etc., prevent the liquid leakage, and is easily produced, and a rechargeable cell comprising the same.
Hitherto, in the production of polymer electrolytes, various polymers such as polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, etc. or monomers which are polymerized with radical polymerization initiators or the irradiation of UV rays are used as gelation agents to gel liquid electrolytes.
However, polyethylene oxide, polyvinylidene fluoride or polyacrylonitrile requires heating to gel the liquid electrolytes. During the heating step, the electrolyte salts may be decomposed. In addition, such polymers have high viscosity and therefore their workability is low.
When the monomers are polymerized with the radical polymerization initiators, heating is usually necessary, and it takes a long time until the polymerization reaction is completed. In addition, the polymerization may not be carried out in the air since radicals are deactivated with oxygen. When the monomers are polymerized with the irradiation of UV ray, the polymerization may proceed at room temperature but it is difficult to produce polymers having the same properties required at any time in a dry air because of the generation of radicals. In addition, the monomers used are mostly polyfunctional, and thus the polymers generated usually becomes fragile easily.
One object of the present invention is to develop a polymer which can be quickly prepared at room temperature in a dry air and is used as a gelation agent for the preparation of a polymer electrolyte.
Another object of the present invention is to provide a polymer electrolyte which can be easily prepared and prevent the liquid leakage from electrochemical equipment.
A further object of the present invention is to provide a rechargeable cell comprising a polymer electrolyte, which can avoid the liquid leakage.
Accordingly, the present invention provides a polymer electrolyte comprising an electrolyte salt, a non-aqueous solvent, and a polymer as a gelation agent which comprises repeating units of the formulas:
xe2x80x94(R1xe2x80x94O)nxe2x80x94,
and
xe2x80x94[CH (R2)xe2x80x94CH2xe2x80x94O]mxe2x80x94
wherein n is 0 or a positive number (nxe2x89xa70) and m is 0 or a positive number (mxe2x89xa70) provided that the sum of n and m is at least 5 (n+mxe2x89xa75), R1 is an alkyl group having 1 to 6 carbon atoms, and R2 is an alkyl group having 1 to 6 carbon atoms or a benzyl group, and a urea structure of the formula (I): 
in which R3 and R4 are the same or different and represent, an alkyl group having 1 to 6 carbon atoms or a hydrocarbon group comprising an aromatic group and having 7 to 11 carbon atoms in total, and R5 is a hydrogen atom, an alkyl group having 1 to 6 carbon atoms or a hydrocarbon group comprising an aromatic group and having 7 to 11 carbon atoms in total, which may be prepared by polymerization through the reaction of an amine compound and a urethane.
Furthermore, the present invention provides a rechargeable cell comprising a positive electrode, a negative electrode and a film of the polymer electrolyte of the present invention, which is interposed between the positive and negative electrodes.
The polymer contained in the polymer electrolyte of the present invention has a decreased crystallinity and a low glass transition temperature because of the introduction of, for example, propylene oxide segments in ethylene oxide segments. Therefore, such a polymer itself has a good ion conductivity even at or below room temperature in comparison with the conventional polymer gelation agents.
Furthermore, the retention of the liquid electrolyte can be controlled by the adjustment of the ratio of the ethylene oxide segments to the propylene oxide segments.
The above polymer used in the present invention is prepared by the polymerization through the reaction of an amine compound and a urethane. This reaction can be carried out in a dry air since it is a polyaddition reaction in which no radicals are generated as intermediates. Thus, the production process of the polymer electrolyte can be simplified.
In addition, the reaction of the active hydrogen of the amine and the isocyanate group of the urethane can be carried out at room temperature, since it proceeds very quickly. The reaction rate can be controlled easily using the difference of the reactivity of a primary amine, a secondary amine and a polyamine and/or the content of the amino group.
In the polymer structure of the present invention, the structures represented by xe2x80x94(R1xe2x80x94O)nxe2x80x94, and xe2x80x94[CH(R2)xe2x80x94CH2xe2x80x94O]mxe2x80x94 are derived from either the amine compound or urethane. The group R3 in the urea structure represented by the formula (I) is derived from the urethane, while R4 and R5 are derived from the amine compound. Thus, those having the above structures are selected as the amine comound and the urethane to synthesize the polymer used in the present invention.
The amine compound used is preferably a primary amine, a secondary amine or polyaminoamide having a polymeric chain of an alkylene oxide, or their derivatives, which preferably has at least two amino groups. Specific examples of such an amine compound include Jeffamine (trade name) comprising polypropylene glycol the both terminals of which are changed to amine groups, diethylene glycol bispropylamine comprising a linear aliphatic diamine into which an ether group is introduced [H2N(CH2)pO(CH2CH2O)qxe2x80x94(CH2)pxe2x80x94NH2], etc.
Preferably, the urethane is an aliphatic urethane, an alicyclic urethane, an aromatic urethane or derivatives thereof with a compound comprising polymeric alkylene oxide chain. Preferably, the urethane has at least two isocyanate groups. Specific examples of such a urethane include 2,4-tolylene diisocyanate (TDI), 4,4xe2x80x2-diphenylmethane diisocyanate (MDI), dianisidine diisocyanate, tolylene diisocyanate, m-xylene diisocyanate, hexamethylene diisocyanate, polymethylenepolyphenyl polyisocyanate, and their addition products with polyols.
Examples of the electrolyte salt are halides and perchlorates of alkali metals, perchlorates of alkaline earth metals, salts of fluorine-containing compounds such as salts of trifluoromethanesulfonic acid, and mixtures thereof. Specific examples of such an electrolyte salt include sodium fluoride, sodium chloride, sodium iodide, lithium bromide, lithium perchlorate, lithium trifluoromethanesulfonate, lithium tetraborofluoride, lithium bistrifluoromethylsulfonylimide, lithium thiocyanate, magnesium perchlorate, magnesium trifluoromethanesulfonate, sodium tetraborofluoride, etc.
Specific examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, xcex3-butyrolactone, dimethoxyethane, dimethylsulfoxide, dioxolane, sulfolane, methylethyl carbonate, etc. They may be used singly or in a mixture.
The polymer electrolyte of the present invention may be produced by any preparation method used to produce conventional polymer electrolytes using the above components.
Also the amounts of the polymer, the electrolyte salt and the non-aqueous solution may be the same as those for the conventional polymer electrolytes.
The rechargeable cell of the present invention has the same construction as that of the conventional rechargeable cell comprising a polymer electrolyte. That is, the rechargeable cell comprises a positive electrode, a negative electrode, and the film of the polymer electrolyte of the present invention.
The active material of the positive electrode can be lithium-containing composite oxides such as lithium cobalt composite oxides (e.g. LiCoO2, etc.), lithium manganese composite oxides (e.g. LiMn2O4, etc), lithium nickel composite oxides (e.g. LiNiO2, etc.), and those derivatives in which a part of the metals are replaced with other metals (e.g. LiNi0.7Co0.2Al0.1O2, etc.) Other metal oxides (e.g. manganese dioxide, vanadium pentoxide, chromium oxide, etc.) or metal sulfides (e.g. titanium disulfide, molybdenum disulfide, etc.) are also usable as the active material of the positive electrode. Furthermore, sulfur element (e.g. polysulfur) or organic sulfur compounds having a disulfide bond may be used as the active material of the positive electrode.
The positive electrode may be prepared by any conventional method. For example, it can be prepared by adding optionally a binder (e.g. polyvinylidene fluoride, polytetrafluoroethylene, etc.) and conductive materials (e.g. flake-form graphite, acetylene black, carbon black, etc.) to the active material of the positive electrode to obtain a positive electrode mixture, then adding a dispersion solvent to the mixture to form a uniform paste, applying the paste on a positive electrode collector (e.g. an aluminum or nickel foil, etc.) and drying the applied paste to form the layer of the positive electrode mixture on at least a part of the positive electrode collector. The binder may beforehand be dissolved in a solvent and then mixed with the active material of the positive electrode and so on.
Examples of an active material of the negative electrode include metal lithium or lithium alloys, carbonaceous materials such as graphite, cokes, mesocarbon microbeads, carbon fiber, activated carbon, etc., alloys or oxides of Si, Sn, In, etc., lithium-containing composite nitrides (e.g. Li2.6Co0.4Ni0.1N, etc.), and so on. Among them, metal lithium, the lithium alloys, and the lithium-containing composite nitrides are preferably used to design high capacity cells.
The negative electrode may be produced by any conventional method. For example, it can be produced by adding optionally the conductive material and the binder to the active material of the negative electrode like in the production of the positive electrode to obtain a negative electrode mixture, dispersing the negative electrode mixture in a solvent to form a uniform paste by stirring, applying the paste on a negative electrode collector (e.g. a copper foil, etc.) and drying the applied paste to form the layer of the negative electrode mixture on at least a part of the negative electrode collector. The binder may be beforehand dissolved in a solvent and then mixed with the active material of the negative electrode and so on.